Joonmyoung Lee

Excellent Selector Characteristics of Nanoscale

We herein present a nanoscale vanadium oxide (VO₂) device with excellent selector characteristics such as a high on/off ratio (>; 50), fast switching speed (<; 20 ns), and high current density (>; 10⁶ A/cm2). Owing to extrinsic defects, a large-area device with a 20-nm-thick VO₂ layer underwent an electrical short. In contrast, after scaling the device active area (<; 5 × 10⁴ nm²), excellent switching uniformity was obtained. This can be explained by the reduced defects and the metal-insulator transition of the whole nanoscale VO₂. By integrating a bipolar resistive random access memory device with the VO₂ selection device, a significantly improved readout margin was obtained. The VO₂ selection device shows good potential for cross-point bipolar resistive memory applications.

\hbox{VO}_{2}

This paper describes the resistive switching of a cross-point cell array device, with a junction area of 100 nm x 100 nm, fabricated using ultraviolet nanoimprinting. A GdO(x) and Cu-doped MoO(x) stack with platinum top and bottom electrodes served as the resistive switching layer, which shows analog memory characteristics with a resistance ratio greater than 10. To demonstrate a neural network circuit, we operated the cell array device as an electrically modifiable synapse array circuit and carried out a weighted sum operation. This demonstration of cross-point arrays, based on resistive switching memory, opens the way for feasible ultra-high density synapse circuits for future large-scale neural network systems.

for High-Density Bipolar ReRAM Applications

We report a simple metal-insulator-metal (MIM)-type selection device that can alleviate the sneak current path in cross-point arrays. By connecting a nanometer-scale Pt/TiO2/TiN selection device to a Pt/TiO2−x/TiO2/W resistive random access memory (RRAM), we could significantly reduce read disturbance from unselected memory cells. This selection device could be easily integrated into an RRAM device, in which it suppressed the sneak current and significantly improved the readout margin compared to that obtained for an RRAM not using a selection device. The introduction of this MIM device can fulfill the requirement for an appropriate selection device for bipolar-type RRAM cross-point applications.

An electrically modifiable synapse array of resistive switching memory

We have investigated the bilayer structure of binary oxides such as HfOx and ZrOx for applications to resistance memory. The ZrOx/HfOx bilayer structure shows a lower reset current and operating voltage than an HfOx monolayer under dc sweep voltage. Furthermore, the bilayer structure exhibits a tight distribution of switching parameters, good switching endurance up to 105 cycles, and good data retention at 85 °C. The resistive switching mechanism of memory devices incorporating the ZrOx/HfOx bilayer structure can be attributed to the control of multiple conducting filaments through the occurrence of redox reactions at the tip of the localized filament.

TiO2-based metal-insulator-metal selection device for bipolar resistive random access memory cross-point application

We report excellent switching uniformity and reliability of RRAM device with ZrOx/HfOx bi-layer films. Precise control of the oxygen vacancy concentration in HfO2 layer was achieved by depositing thin Zr metal (2–15nm) layer. Scaling down active device area (ϕ=50 nm) and film thickness (&#60;2–5 nm) can significantly minimize the extrinsic defects-related non-uniform switching which was normally observed in large area (ϕ >um) device, with higher active layer thickness (>10 nm). Using back-to-back connection of two RRAM devices, we confirmed feasibility of a diode-free cross-point array with a wide readout margin and stable data reading. Considering excellent electrical and reliability characteristics of diode-free RRAM device, shows a great promise for future high density cross-point memory devices

Effect of ZrOx/HfOx bilayer structure on switching uniformity and reliability in nonvolatile memory applications

We have investigated a Cu-doped MoO_x/GdO_x bilayer film for nonvolatile memory applications. By adopting an ultrathin GdO_x layer, we obtained excellent device characteristics such as resistance ratio of three orders of magnitude, uniform distribution of set and reset voltages, switching endurance up to 10⁴ cycles, and ten years of data retention at 85degC. By adopting bilayer films of Cu-doped MoO_x/GdO_x, a local filament was formed by a two-step process. Improved memory characteristics can be explained by the formation of nanoscale local filament in the ultrathin GdO_x layer.

Diode-less nano-scale ZrO x /HfO x RRAM device with excellent switching uniformity and reliability for high-density cross-point memory applications

We demonstrated multibit operation using a 250-nm Ir/TiOx/ TiN resistive random access memory by Schottky barrier height engineering. A Schottky barrier was formed by the interface between a high-work-function Ir top electrode and n-type TiOx. The conducting path, which was composed of oxygen vacancies, was generated in a low-resistance state, whereas a Schottky barrier was reproduced in a high-resistance state (HRS) due to the high concentration of oxygen by the electric field. By changing the reset operation voltage, we successfully engineered the Schottky barrier height, resulting in the modulation of the HRS current and demonstrating the feasibility of multibit applications.

Excellent Switching Uniformity of Cu-Doped

A systematic study on the switching mechanism of an Al/ Pr0.7Ca0.3MnO3 (PCMO) device was performed. A polycrystalline PCMO film was deposited using a conventional sputtering method. A thin Al layer was introduced to induce a reaction with the PCMO, forming aluminum oxide (AlOx). Transmission electron microscopy analysis of the interface between Al and PCMO showed that resistive switching was governed by the formation and dissolution of AlOx. Some basic memory characteristics, such as good cycle endurance and data retention of up to 104 s at 125°C, were also obtained. It also showed excellent switching uniformity and high device yield.

\hbox{MoO}_{x}/\hbox{GdO}_{x}

We investigated the state stability of the low-resistance state (LRS) in a resistive switching memory having a Pt/Cu:MoOx/GdOx/Pt structure. Various resistance values of LRS were accurately controlled using an external load resistor connected in series with the resistive memory device. We found that the retention time decreased with an increase in the resistance of LRS. We performed accelerating tests for resistance transition from a low- to a high-resistance state under temperatures ranging from 200°C to 250°C. A predicted resistance of LRS for a ten-year retention period at 85°C was determined based on the Arrhenius law.

Bilayer for Nonvolatile Memory Applications

We have investigated copper-doped carbon (CuC) as a new solid-state electrolyte material for resistive switching devices. Compared with CuS electrolytes, CuC devices demonstrate good memory characteristics such as a high resistance ratio of over two orders, higher operation voltage, and high temperature retention characteristics. Using 1000 cell array devices, we have also confirmed uniform distributions of resistance and switching voltages. Both high and low resistance states showed negligible degradation of resistance for over 104 s at 85 °C, confirming good retention characteristics.